Method of laser connection of a conductor to a doped region of the substrate of an integrated circuit

ABSTRACT

The conductor 15 to be connected to the doped region 12 of the substrate 11 has an edge 15a at which the laser beam 20 is aimed, regulated such as to definitively create a zone of low electrical resistance 19 in the dielectric layer 13 that separates the conductor from the doped region. The invention is particularly applicable to programming by laser of read only memories and defective integrated circuits with a view to correcting them.

This is a continuation of application Ser. No. 214,466, filed Jul. 1,1988, now abandoned.

FIELD OF THE INVENTION

The invention relates to a method of connecting a conductor to a dopedregion of a substrate of an integrated circuit using a laser, and anintegrated circuit utilizing the method.

BACKGROUND OF THE INVENTION

An integrated circuit, currently known as a chip, is essentially asemiconductor substrate including doped regions for defining electroniccomponents and having an interconnect structure for these components.The interconnect structure rests on the substrate via a dielectricsilicon dioxide (SiO2) layer formed by growth of the material of thesubstrate. Generally, the dielectric layer is thin (several tens ofnanometers thick) above the doped regions, and thick (several nanometersthick) between these regions. The interconnect structure presentlycomprises a plurality of conductive layers separated by insulatinglayers and connected to one another at certain points by vias passingthrough the insulating layers. Each conductive layer comprises numerousconductors parallel to one another in a direction orthogonal to theconductors of a neighboring conductive layer. Conductors of the lowerlayer are connected with the corresponding regions of the substrate viaopenings made in the thin dielectric layer covering these regions.

At present in the manufacture or design of very large scale integratedcircuits, or VLSI chips, attempts are being made to make wide use of thelaser to modify the circuits on these chips. In particular, modificationof the circuits of a chip is done to correct certain connection defects.Connections made by laser are then programmed for reconfiguring acircuit because of a defect, or to connect a substitute circuit to adefective circuit in accordance with the technique known as redundancyof the functional blocks of an integrated circuit. On the other hand,modification of the integrated circuits in a chip naturally takes placewhen the chip is, or includes, a programmable read only memory (PROM).The conduction state of the cells of the memory is determined by one ormore connections between predetermined conductor elements of each cell.The use of the laser is one solution intended for programming a PROM. Inconclusion, laser programming of the connections in the integratedcircuits of a chip has numerous applications and numerous advantages.

Several methods are now known for making an electrical connection of twosuperimposed conductors of an integrated circuit, or between a conductorand a region of the substrate, at a point through the thin dielectriclayer by laser beam.

One known method of programmable bonding by laser of two superimposedconductors is described in particular in the article by N. S. Platakisin the Journal of Applied Physics, Vol 47, No. 5, May, 1976, pp.2120-2128. At the desired bonding point, a laser beam is applied, theenergy, diameter, number and duration of the pulses of which aredetermined so as to progressively open the upper conductor, then thedielectric layer and, partially, the lower conductor and to form acontact between the two conductors by ejecting melted material of thelower conductor up to the level of the upper conductor. Then the twoconductors are connected to one another by the solidified ejections onthe walls of a crater, which thus has the approximate shape of ametallized hole. This method has numerous disadvantages.

First, it will be understood that the uncontrollable ejections of meltedmaterial of the lower conductor placed beneath a laser beam, underidentical conditions, yield different configurations of the band of theconductors in each crater. As a result, the bonds formed by this method,under identical conditions, have different electrical connectionqualities. Experience has confirmed the defects in terms ofreplicability and reliability of this method.

As a second disadvantage, the formation of a crater requires the use ofa powerful laser beam and damages the structure of the two conductorelements and the insulating layer separating them. On the one hand, thethermal shock can create electrical defects in the active elements(junctions, transistors, and so forth) which surround the crater. On theother hand, the thermal shock creates a dislocation of the structure ofthe conductor elements, which is particularly undesirable when the lowerconductor element is a doped region of a silicon substrate. Even if thelower element is a conductor of the interconnect structure, this elementmust have a minimum thickness so as not to damage the insulating layerthat separates it from the substrate. Consequently the connection bylaser, in the form of a crater, is limited in practice to the connectionof two conductor elements of the interconnect structure formed above asilicon substrate. It follows that for a desired connection between anupper conductor and a doped region of the substrate, this region must beconnected to an intermediate conductor, with which the upper conductorwill be connected by a crater sufficiently distant from the doped regionof the substrate.

Another disadvantage of this connection method is that it cannot be doneafter the manufacture of the integrated circuit. The manufacture of anintegrated circuit ends with the covering of the entire interconnectstructure with a thick passivation layer adapted for protecting theintegrated circuit electrically, mechanically and chemically. It will beunderstood then that laser connection cannot be done correctly exceptprior to the deposition of the passivation layer, that is, during theprocess of manufacture of the integrated circuit. This disadvantagetranslates into a lack of flexibility in programming the bonds in theintegrated circuit and an increase in cost.

Finally, connection at one point by laser in the form of a crater iseasily observable by optical microscope, and an observer can learn theentire configuration of the connections made on an integrated circuitfrom it. This kind of connection thus cannot be used when theconnections are part of confidential information or that are to remaininviolable, as is the case for example for integrated circuit memoriesin credit cards (known as smart cards).

Another known method for programmable bonding by laser of twosuperimposed conductors is described in particular in the article by J.I. Raffel et al, entitled "Laser Programmed Vias for RestructurableVLSI" in the publication "Technical Digest of the International ElectronDevices Meeting", 1980, pp. 132-135. This method comprises usingamorphous silicon in the dielectric layer at the level of the bondingpoint. The use of amorphous silicon in predetermined regions of thedielectric layer has the disadvantage of complicating the method ofmanufacturing the integrated circuit and of prohibiting programming anybonds by laser outside these regions.

The invention proposes a method of connection by laser of a conductorelement to a doped region of the substrate of an integrated circuit,which has the advantage over the prior methods of being simple, reliableand effective, with a less powerful laser, which is usable during andafter the manufacture of the integrated circuit and leaves practicallyno trace that is detectable by microscope of the connection that hasbeen made.

SUMMARY OF THE INVENTION

The method according to the invention of electrical connection by laserof a conductor to a doped region of the substrate of an integratedcircuit by the intermediary of a thin dielectric layer comprisesdisposing one edge of the conductor above the region, causing the laserbeam to straddle the edge of the conductor and the region, andregulating the power, the diameter, the number and duration of thepulses of the laser beam such as to definitively create defects, in onezone of the dielectric layer, which lend it a low electrical resistance.

The characteristics and advantages of the invention will become apparentfrom the ensuing detailed description of a preferred embodiment shown inthe drawing, which is understood to be solely exemplary and not in anyway limiting to the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The single drawing FIGURE is a cross-sectional view of an integratedcircuit showing the connection by laser according to the invention of aconductor of the interconnect structure to a doped region of thesemiconductor substrate of the integrated circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning to the drawing, the integrated circuit 10 substantially includesa monocrystalline semiconductor substrate 11, typically of silicon, theupper surface 11a of which includes doped regions 12. The drawing showsa fragmentary sectional view of the integrated circuit 10 at the levelof a doped region 12. In the drawing, the proportions of the length havenot been drawn to scale, for the sake of simplicity in the drawing, andwill be readily corrected appropriately by one skilled in the art. Adielectric layer 13 of silicon dioxide (SiO2) is formed by oxidation ofthe silicon of the substrate 11 over its entire surface 11a. At thelevel of the doped regions 12, the dielectric layer is ordinarily verythin, on the order of several tens of nanometers. An interconnectstructure 14 of the regions 12 of the substrate 11 is formed on thedielectric layer 13. In the conventional manner, the network 14 showncomprises lower conductors 15 and upper conductors 16 separate from oneanother by an insulating layer 17. The lower conductors 15 may bemetallic, ordinarily of aluminum, or may be made of polycrystallinesilicon also known as polysilicon, suitably doped so as to beconductors, or may comprise a metal-silicon alloy. The upper conductors16 are typically of metal, such as aluminum. The example shown relatesto the use of the invention after the manufacture of the integratedcircuit 10, which in other words has already been provided with apassivation layer 18 covering the interconnect structure 14 to protectit electrically, mechanically and chemically. The passivation layer 18is ordinarily made of a glass doped with phosphorus.

To make the connection 19 of a conductor 15 to a region 12 by means of alaser beam 20, the method according to the invention comprises, first,disposing one edge of the conductor 15 above the region 12. In theexample shown, it is the end 15a of the conductor 15 that is placed atthe region 12 and is thus considered as an edge in the context of theinvention. However, this may instead be a long edge of the conductor.The method next comprises using a laser beam 20 having a diameter B, apower P and a pulse duration T. The laser beam 20 is aimed orthogonallyat the upper face 11a of the substrate 11, such that its target zone 21is divided into a zone 21a on the edge 15a of the conductor 15 and azone 21b on the dielectric layer 13. The target zone 21 is then placedstraddling the conductor 15 and the dielectric layer 13 in the region12. Finally, the power P, the diameter D, the number N and the durationT of the pulses of the laser beam 20 are adjusted such as todefinitively create defects in the zone 19 of the dielectric layer thatlend it a low electrical resistance. The connection zone 19substantially corresponds to the zone 21 covered by the beam 20. The lowelectrical resistance obtained under various experimental conditionsextends over a range between approximately 100Ω and several kΩ moreparticularly between 300Ω and 3kΩ.

By way of example, the following typical experiment was performed withan integrated circuit the silicon dioxide dielectric layer 13 of whichhad a thickness of 70 nm, the region 12 had a depth of 400 nm, thepolysilicon conductor 15 had a thickness of 500 nm, and the insulatinglayer covering it was approximately one micrometer in thickness. Thelaser beam 20 had a diameter D of 5 μm and delivered a power P of 0.76watts during a single pulse (N=1) of duration T=1 ms. The resistancemeasured between the conductor 15 and the region 12 rose toapproximately 350Ω.

The resistance of the connection 19 depends on the parameters of use ofthe beam, the dimensions of the elements through which the beam passes,and their type. A metal conductor 15 is suitable. However, it has beenobserved that if the conductor 15 is made of doped polycrystallinesilicon of a given type, the connection 19 according to the inventioncannot exist except if the region 12 and the conductor 15 have the sametype of doping. Hence the type of doping of the polysilicon conductor 15must be selected, or a sufficient existing region must be located overthe substrate, in order to make a connection 19 according to theinvention; or one must even create such a region, or append to theconductor 15 a metal conductor that makes it possible to make theconnection 19.

The first advantage of the invention is that a reliable and effectiveconnection is made in a simple manner with a laser beam of very lowenergy by comparison with the indispensable energy required to form acrater in accordance with the prior art.

The second advantage of the invention is the fact that the connectionzone 19 according to the invention is practically undetectable by theoptical microscope. It is thus assumed that the low energy brought tobear by the laser beam is utilized by the thin dielectric layer 13 tocreate definitive defects, with the surrounding conductive material,which are invisible yet sufficient for obtaining a low electricalresistance. It has been demonstrated that the connection zone 19 extendsonly within the thin dielectric layer and when viewed in an opticalmicroscope practically does not change the conductor elements connectedaccording to the invention.

The third advantage of the invention is that a connection is madeindependently of the conductor elements and insulating elements 15, 16,17, 18 successively covering the thin dielectric layer. Since a very lowenergy at the level of this layer is sufficient, the energy at theoutlet of the laser, taking into account the absorption of energy in thepreceding elements, remains low.

In any case, the energy absorbed in these elements is incapable ofmodifying their nature or their structure.

On the other hand, it should be noted that the components of theintegrated circuit can be made of doped regions of the substrate andconductors formed above the substrate, in the vicinity of these regionsand by the intermediary of thin dielectric layers. This is the case, forexample, with MOS transistors (metal oxide semiconductor transistors),the gate of which is made of a conductive strip extending above thesubstrate and separating the two doped regions from the drain and sourceof the substrate. The gate conductor may be of metal or of highly dopedpolycrystalline silicon. The invention equally relates to a conductor ofthat type. For example, by lengthening the gate of an MOS transistor,the invention makes it possible to connect the gate by laser directly toa neighboring region that is either independent of or a constituent partof some other component.

It will now be understood that the application of the invention toprogramming by laser of connections of integrated circuits is highlyadvantageous and very widely applicable. In particular, programming bylaser in accordance with the invention will be highly esteemed forcorrecting integrated circuits in VSLI chips or for programming ofPROMs.

What is claimed is:
 1. A method of electrical connection by laser of aconductor (15) to a doped region (12) of a substrate (11) of anintegrated circuit (10) by the intermediary of a thin dielectric layer(13), comprising disposing one edge (15a) of the conductor above thedoped region, causing a laser beam (20) to straddle (21, 21b) the edgeof the conductor adjacent the doped region, and regulating the power(P), the diameter (D), the number (N) and duration (T) of the pulses ofthe laser beam so as to definitively create defects in the dielectriclayer without substantially changing a structure of said conductor andof said doped region of said substrate exposed to said laser beam,thereby lowering an electrical resistance of said dielectric layermaterial in a zone exposed to said laser beam.
 2. A method as defined byclaim 1, wherein the conductor is of metal.
 3. A method as defined byclaim 1, wherein the doped region of the substrate has dopants of oneconductivity type and the conductor includes a semiconductor materialhaving dopants of said one conductivity type.
 4. A method as defined byclaim 3, wherein said semiconductor material is made of polycrystallinesilicon.
 5. A method as defined by claim 3, wherein said semiconductormaterial is made of metallic material and polycrystalline silicon.
 6. Amethod as defined by claim 1, comprising the further step of coveringthe substrate with at least one additional conductor layer.
 7. A methodas defined in claim 6 wherein the step of covering the substrate withsaid at least one additional conductor layer is performed prior to saidstep of electrically connecting by laser said conductor to said dopedregion.
 8. A method as defined by claim 1 comprising the further step ofcovering the substrate with at least one insulating layer.
 9. A methodas defined in claim 8 wherein the step of covering the substrate withsaid at least one insulating layer is performed prior to said step ofelectrically connecting by laser said conductor to said doped region.10. A method as defined in claim 1 comprising the further steps ofcovering the conductor with at least one insulating layer, andsubsequently depositing at least one additional conductor layer on saidinsulating layer.
 11. A method as defined in claim 10 wherein said stepsof covering said conductor with at least one insulating layer anddepositing at least one additional conductor layer on said insulatinglayer are performed prior to said step of electrically connecting bylaser said conductor to said doped region.